Satellites in geostationary orbits (GSO's) have been widely preferred because of the economic advantages afforded by such orbits. In a geostationary orbit, a satellite traveling above the earth's equator, in the same direction as that in which the earth is rotating, and at the same angular velocity, appears stationary relative to a point on the earth. These satellites are always "in view" at all locations within their service areas, so their utilization efficiency is effectively 100 percent. Antennas on earth need be aimed at a GSO satellite only once; no tracking system is required.
Given the desirability of geostationary satellite orbits and the fact that there are only a finite number of available "slots" in the geostationary "belt," the latter capacity has been essentially saturated with satellites operating in desirable frequency bands up through the Ku-band (up to 18 GHz). As a result, the government has been auctioning the increasingly scarce remaining slots.
This has encouraged the development of complex and expensive new systems including those using low earth orbits (LEO's), medium earth orbits (MEO's), and higher frequencies, for example, the Ka and V-bands (up to approximately 50 GHz). Growth to higher frequencies is limited by difficult problems of technology and propagation, and expansion in satellite applications requires exploitation of the spatial dimension (i.e., above and below the GSO belt). A host of proposed LEO and MEO systems exemplify this direction.
For LEO satellites, however, larger beams are required at the center of coverage and smaller beams near the edges of the coverage to compensate for the path length differences. In addition, the beams are required to be circular close to the center of coverage and elliptical at the edge of coverage for a uniform cell size on the earth. The different beam requirements increase the complexity of the beam-forming circuitry.
In known satellite systems, signals from each feed are divided into a number of beam portions. Each portion is amplitude and phase weighted using variable active components. The beam portions are then combined to form beams. The feed network for the known systems becomes quite complicated because a large dividing network, a large combining network and large number of variable attenuators and/or variable phase shifters are required. The number of variable attenuators is the product of the number of beams and the number of elements per beam.
Weight, size and power consumption are always a concern with satellite designs. The beam-forming network is complex and thus the weight and size and power consumption are relatively high. It would therefore be desirable to reduce the complexity of the beam-forming network and therefore reduce the size, weight and power consumption of the satellite.